An optical interface system for a short-wave infrared on-chip spectrometer and a method of manufacturing the same

Abstract

This invention discloses an optical interface system and its fabrication method for short-wave infrared on-chip spectrometers, belonging to the field of optical measurement and spectral analysis technology. The system includes a polarization-maintaining fiber, a tapered lens structure, an inverted tapered mode converter, and a packaging and fixing structure. The polarization-maintaining fiber core material is chalcogenide glass with an elliptical cross-section; the tapered lens structure is located at the end of the polarization-maintaining fiber, with a taper angle smaller than the critical angle of the adiabatic tapered lens; the inverted tapered mode converter is integrated into the edge of the on-chip spectral analysis chip, with its wide end matching the output mode field size of the tapered lens structure and its narrow end matching the mode field size of the silicon waveguide; the packaging and fixing structure fixes the polarization-maintaining fiber relative to the chip. This invention also provides a method for fabricating this optical interface system. This invention achieves low-loss transmission, high polarization-maintaining performance, and efficient fiber-chip coupling in the short-wave infrared band, providing a complete optical interface solution for the practical deployment of computational reconstruction on-chip spectrometers in the short-wave infrared band.

Description

Technical Field

[0001] This invention relates to the field of optical measurement and spectral analysis technology, and in particular to an optical interface system for short-wave infrared on-chip spectrometers and its fabrication method. Background Technology

[0002] The short-wave infrared band (1.8μm–2.5μm) covers the characteristic absorption bands of various gases and organic molecules, including water molecules, carbon dioxide, and methane, and has significant application value in environmental gas detection, food quality identification, and biomedical spectral imaging. Computational reconstruction on-chip spectrometers are a key technological path for achieving miniaturization and integrated deployment of these applications. Their working principle is as follows: the optical signal to be measured is transmitted via optical fiber to an on-chip spectral analysis chip. After spectral encoding and sampling are completed within the chip, the signal is received by a detector array, and the spectral information is reconstructed by a back-end algorithm. In this optical link, the transmission characteristics of the optical fiber and the coupling interface performance between the fiber and the chip directly determine the effective optical power entering the chip, polarization stability, and sampling uniformity across the entire band, thus affecting the signal-to-noise ratio and accuracy of the final spectral reconstruction.

[0003] However, when the operating wavelength extends from near-infrared to short-wave infrared (1.8μm~2.5μm), existing optical interface solutions face the following systemic technical bottlenecks: First, quartz optical fibers suffer from fundamental transmission loss limitations in this wavelength band. Beyond 1.8 μm, the transmission loss of quartz glass increases dramatically due to multiphonon absorption from silicon-oxygen bond vibrations, severely reducing the signal strength reaching the detector. Although non-oxide materials such as chalcogenide glasses exhibit excellent low-loss characteristics in the short-wave infrared, current solutions have not yet systematically applied them to the optical interface design of on-chip spectrometers.

[0004] Secondly, conventional single-mode optical fibers lack polarization maintenance capabilities. Silicon waveguides exhibit significant structural asymmetry and are extremely sensitive to the polarization state of input light; the transmission characteristics of TE and TM modes differ drastically from their filtering responses. When ordinary single-mode optical fibers are bent, compressed, or subjected to temperature changes, their polarization states evolve randomly, leading to uncontrollable polarization states of the light coupled into the chip, causing distortion in the filtering response and a decrease in spectral reconstruction accuracy. Existing stress-type polarization-maintaining fibers (such as panda-type and bowtie-type fibers) are extremely difficult to fabricate in non-quartz glass systems.

[0005] Third, the huge size mismatch between the fiber mode field and the silicon waveguide mode field leads to extremely low coupling efficiency. The mode field diameter of special glass fiber is on the order of micrometers, while the mode field size of silicon nanowire waveguide is on the order of submicrometers, a difference of one to two orders of magnitude, resulting in extremely low direct end-face coupling efficiency. In existing solutions, grating couplers have narrow operating bandwidths and are sensitive to polarization; lens fiber solutions are limited by the single-level mode field compression range, resulting in poor consistency over a wide band; and none of the above solutions have been optimized in conjunction with the system requirements of on-chip spectrometers.

[0006] In the field of special polarization-maintaining fibers, existing studies have reported the optical transmission characteristics of chalcogenide glass elliptical core polarization-maintaining fibers and microstructure polarization-maintaining fibers in the mid-infrared band. However, the above-mentioned work has mostly focused on the transmission characteristics of the fiber itself or the fiber manufacturing method. A complete technical solution has not yet been formed that systematically combines elliptical core polarization-maintaining special glass fibers with silicon substrate waveguide devices for joint design and synergistic optimization of the entire short-wave infrared link.

[0007] In summary, the existing technology lacks a systematic optical interface solution that can simultaneously solve the problems of low-loss fiber transmission, polarization preservation, and efficient fiber-chip coupling in the short-wave infrared band, making it difficult to meet the technical requirements for the practical deployment of computational reconstruction on-chip spectrometers in the short-wave infrared band. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides an optical interface system and its fabrication method for short-wave infrared on-chip spectrometers, aiming to achieve efficient and polarization-stable transmission of short-wave infrared optical signals from optical fiber to the chip, and to provide a complete optical interface solution for the practical deployment of computational reconstruction on-chip spectrometers in the short-wave infrared band.

[0009] To achieve the above objectives, the technical solution of the present invention is as follows: An optical interface system for a short-wave infrared on-chip spectrometer includes: The polarization-maintaining fiber has a core material of chalcogenide glass and an elliptical core cross-section. The operating wavelength range of the polarization-maintaining fiber covers 1.8 μm to 2.5 μm. The mode birefringence of the polarization-maintaining fiber is generated by the geometric asymmetry of the elliptical core and is greater than 10. -5 ; A tapered lens structure is disposed at the end of the polarization-maintaining fiber. The tapered lens structure is formed by heating and stretching the end of the polarization-maintaining fiber to form a tapered transition region. The cone angle of the tapered transition region is smaller than the critical angle of the adiabatic tapered fiber, so that the fundamental mode optical field energy adiabatically follows the change of the waveguide cross-sectional size within the tapered transition region, suppressing leakage to higher-order modes or radiation modes. A microlens surface is formed at the end of the tapered transition region. An inverted conical mode converter is integrated into the edge coupling region of an on-chip spectral analysis chip. The width of the inverted conical mode converter gradually increases from the narrow end to the wide end along the light propagation direction. The mode field size at the wide end matches the output mode field size at the end of the tapered lens structure, and the mode field size at the narrow end matches the mode field size of the silicon waveguide in the on-chip spectral analysis chip. The encapsulation and fixing structure fixes the polarization-maintaining fiber relative to the on-chip spectral analysis chip, so that the end of the tapered lens structure is optically aligned with the wide end of the inverted conical mode converter.

[0010] In the above scheme, the chalcogenide glass is As2S3, the major axis radius of the elliptical core is 6μm, the minor axis radius is 2μm, the cladding radius is 40μm, the core refractive index is 2.42, the cladding refractive index is 2.408, and the mode birefringence of the polarization-maintaining fiber at a wavelength of 2μm is 2.28×10⁻⁶. -5 The birefringence of the mode is generated by the geometric asymmetry of the elliptical fiber core and does not depend on the heterogeneous stress region around the fiber core.

[0011] In the above scheme, the inverted conical mode converter is fabricated from silicon-based waveguide material by deep ultraviolet lithography and dry etching processes. The length of the inverted conical mode converter meets the adiabatic mode field conversion conditions in the full band range of 1.8μm to 2.5μm. The edge end face of the on-chip spectral analysis chip is provided with an antireflection film covering the 1.8μm to 2.5μm band.

[0012] In the above scheme, the tapered lens structure of the polarization-maintaining fiber and the inverted conical mode converter of the on-chip spectral analysis chip constitute a two-stage joint mode field conversion structure. The tapered lens structure performs the first-stage mode field conversion, gradually compressing the fundamental mode field of the polarization-maintaining fiber from the micrometer level to the intermediate mode field size. The inverted conical mode converter performs the second-stage mode field conversion, further compressing the intermediate mode field size to match the submicrometer mode field size of the silicon waveguide. The two-stage joint mode field conversion structure achieves flattening of coupling efficiency across the entire 1.8μm to 2.5μm wavelength range.

[0013] In the above scheme, the encapsulation and fixing structure includes: Fiber optic clamps are used to fix the polarization-maintaining fiber. A chip stage for supporting the on-chip spectral analysis chip; A rigid fastener is disposed at the end of the polarization-maintaining fiber near the on-chip spectral analysis chip. The thermal expansion coefficient of the rigid fastener matches that of the chalcogenide glass. The dimension of the rigid fastener along the fiber length is less than or equal to 3 mm. The rigid fastener is a Kovar alloy V-groove clamp. An elastic fixing material is disposed in the region of the polarization-maintaining fiber away from the on-chip spectral analysis chip to provide flexible constraint on the polarization-maintaining fiber.

[0014] In the above scheme, the chip stage is a copper stage, and the copper stage is fixedly connected to the Kovar alloy V-groove fixture through a Kovar alloy connector; a refractive index-matched curing adhesive is provided between the end face of the tapered lens structure and the edge face of the on-chip spectral analysis chip. The curing adhesive is transparent in the 1.8μm to 2.5μm band and is a low-shrinkage UV curing adhesive, which is used to suppress Fresnel reflection and optically couple the tapered lens structure to the inverted conical mode converter.

[0015] A method for fabricating an optical interface system for a short-wave infrared on-chip spectrometer includes the following steps: Step 1: Fabrication of polarization-maintaining fiber. The core material of the polarization-maintaining fiber is chalcogenide glass, and the core cross-section of the polarization-maintaining fiber is elliptical with a mode birefringence greater than 10. -5 The operating wavelength covers 1.8μm to 2.5μm; Step 2: The end of the polarization-maintaining fiber is heated and stretched to form a tapered lens structure with a tapered transition region. The cone angle of the tapered transition region is smaller than the critical angle of the adiabatic tapered section, so that the fundamental mode optical field energy adiabatically follows the change of the waveguide cross-sectional size within the tapered transition region, suppressing leakage to higher-order modes or radiation modes. The output light power and output spot morphology are monitored in real time to provide feedback control of the tapering parameters. The end of the tapered transition region forms a microlens surface. Step 3: Fabricate an inverted conical mode field converter in the edge region of the on-chip spectral analysis chip. The wide-end mode field size of the inverted conical mode field converter matches the output mode field size at the end of the tapered lens structure, and the narrow-end mode field size matches the mode field size of the silicon waveguide in the on-chip spectral analysis chip. Step 4: Perform six-dimensional precision alignment between the end of the tapered lens structure and the wide end of the inverted conical mode converter to simultaneously achieve optimal values ​​for coupled light power and polarization extinction ratio; Step 5: Fix the polarization-maintaining fiber relative to the on-chip spectral analysis chip using the encapsulation and fixing structure. Inject a low-shrinkage curing adhesive with matching refractive index into the gap between the end face of the tapered lens structure and the edge face of the on-chip spectral analysis chip, and perform ultraviolet curing while maintaining alignment accuracy.

[0016] In the above scheme, in step one, chalcogenide glass ingots are prepared by melt quenching, and then preforms with elliptical fiber cores are prepared by precision extrusion molding. During the drawing process, the drawing tension and temperature distribution are controlled to maintain the axial uniformity of the elliptical fiber core shape, and a low-absorption protective coating suitable for use in the 1.8μm~2.5μm wavelength band is coated after drawing.

[0017] In the above scheme, in step two, a CO2 laser is used to locally heat the end of the polarization-maintaining fiber, and a precision displacement stage is used to control the stretching speed; the end of the tapered transition region is micro-shaped by a focused laser beam to form a microlens surface; in step three, an inverted tapered waveguide is prepared by deep ultraviolet lithography and dry etching processes, the edge face of the on-chip spectral analysis chip is chemically and mechanically polished to optical roughness, and a broadband antireflection multilayer film covering the 1.8μm to 2.5μm band is deposited by magnetron sputtering.

[0018] In the above scheme, step five, the encapsulation and fixation further includes: after UV curing, rigidly fixing the end of the polarization-maintaining fiber near the on-chip spectral analysis chip using a Kovar alloy V-groove clamp with a thermal expansion coefficient matching that of the chalcogenide glass, wherein the dimension of the Kovar alloy V-groove clamp along the fiber length direction is less than or equal to 3 mm; using a low-modulus elastic fixing material for flexible constraint in the remaining area of ​​the polarization-maintaining fiber; the on-chip spectral analysis chip is mounted on a copper stage using a thermally conductive material, and the copper stage and the fiber clamp are fixedly connected by a Kovar alloy connector.

[0019] Through the above technical solution, the optical interface system and its fabrication method for short-wave infrared on-chip spectrometers provided by the present invention have the following beneficial effects: (i) Achieving low-loss transmission across the entire shortwave infrared band. This invention uses chalcogenide glass (As2S3) as the fiber substrate material, whose multiphonon absorption edge is located at a longer wavelength. It can maintain the transmission loss at a low level across the entire operating band of 1.8μm to 2.5μm, fundamentally breaking through the upper limit of the use of quartz optical fiber in the shortwave infrared wavelength and effectively improving the effective optical power reaching the chip.

[0020] (II) Achieving a balance between high polarization-maintaining performance and fabrication process feasibility. This invention employs an elliptical core geometric birefringent structure (major axis radius 6μm, minor axis radius 2μm), achieving a mode birefringence of 2.28 × 10⁻⁶ at a wavelength of 2μm. -5 This solution meets the requirements for polarization-maintaining optical fibers. It eliminates the need for introducing heterogeneous stress regions, completely avoiding the technological challenges of stress region fabrication in chalcogenide glass systems, thus achieving a balance between polarization-maintaining performance and process feasibility.

[0021] (III) Achieving efficient, wide-band, and polarization-stable coupling between fiber and chip. This invention constructs a two-stage joint mode field conversion structure by using a tapered lens at the fiber end and an inverted conical mode field converter at the chip end. This structure gradually compresses the fiber fundamental mode from the micrometer level to match the submicrometer mode field of the silicon waveguide, achieving flattening of the coupling efficiency across the entire 1.8μm to 2.5μm band. This avoids the false spectral baseline distortion introduced by wavelength-dependent coupling efficiency fluctuations.

[0022] (iv) Achieving reliable thermal expansion matching packaging. This invention adopts a composite packaging scheme that combines rigid fixing (≤3mm Kovar alloy V-groove, with thermal expansion coefficient matching that of chalcogenide glass) with flexible constraint (low modulus elastic material), which controls the absolute thermal displacement caused by temperature changes to the submicron level (less than the coupling alignment tolerance), ensuring long-term alignment stability under temperature cycling conditions.

[0023] (v) Provide a complete system-level optical interface solution. This invention forms a complete technical system covering the entire optical interface of short-wave infrared on-chip spectrometers, from material selection, polarization-maintaining structure, mode field conversion to packaging and fixation. The various components work together to provide complete technical support for the practical deployment of computational reconstruction on-chip spectrometers in the short-wave infrared band. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0025] Figure 1 This is a schematic diagram of the optical interface system structure for a short-wave infrared on-chip spectrometer according to the present invention.

[0026] Figure 2 This is a schematic diagram of the encapsulation and fixing structure in the optical interface system for a short-wave infrared on-chip spectrometer as described in this invention.

[0027] In the figure, 100 is the polarization-maintaining fiber; 200 is the tapered lens structure; 300 is the inverted conical mode converter; 400 is the packaging and fixing structure; 410 is the fiber optic clamp; 420 is the chip stage; 430 is the rigid fixing component (Kovar alloy V-groove clamp); 440 is the elastic fixing material; 450 is the refractive index matching curing adhesive; and 500 is the on-chip spectral analysis chip. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0029] This invention provides an optical interface system for short-wave infrared on-chip spectrometers and its fabrication method, with specific embodiments as follows: I. Overall Structure of the Optical Interface System This embodiment provides an optical interface system for shortwave infrared on-chip spectrometers.

[0030] like Figure 1As shown, the optical interface system consists of four parts: polarization-maintaining fiber 100, fiber-end tapered lens structure 200, chip-end inverted tapered mode converter 300, and packaging and fixing structure 400.

[0031] The core material of polarization-maintaining fiber 100 is chalcogenide glass (As₂S₃), with an elliptical core cross-section, covering the operating wavelength range of 1.8 μm to 2.5 μm. The major axis radius of the elliptical core is 6 μm, the minor axis radius is 2 μm, and the cladding radius is 40 μm. The core refractive index is 2.42, the cladding refractive index is 2.408, and the core-cladding refractive index difference Δn ≈ 0.012. The mode birefringence of polarization-maintaining fiber 100 is generated by the geometric asymmetry of the elliptical core, and is 2.28 × 10⁻⁶ at a wavelength of 2 μm. -5 greater than 10 -5 It does not depend on the heterogeneous stress zone around the fiber core.

[0032] A tapered lens structure 200 is disposed at the end of the polarization-maintaining fiber 100, and a tapered transition region is formed by heating and stretching the end of the polarization-maintaining fiber 100 with a CO2 laser. The cone angle of the tapered transition region is smaller than the critical angle of the adiabatic tapered fiber (in this embodiment, the cone angle is less than 5°), so that the fundamental mode optical field energy adiabatically follows the change of the waveguide cross-sectional dimensions within the tapered transition region, suppressing leakage to higher-order modes or radiation modes. A microlens surface is formed at the end of the tapered transition region, and the radius of curvature of the microlens surface is determined according to the convergence requirements of the output optical field.

[0033] An inverted conical mode converter 300 is integrated into the edge coupling region of the on-chip spectral analysis chip. The width of the inverted conical mode converter 300 gradually increases from the narrow end to the wide end along the light propagation direction. The mode field size at the wide end matches the output mode field size at the end of the tapered lens structure 200, and the mode field size at the narrow end matches the mode field size of the silicon waveguide in the on-chip spectral analysis chip. The inverted conical mode converter 300 is fabricated from silicon-based waveguide material using deep ultraviolet lithography and dry etching processes, and its length meets the adiabatic mode field conversion conditions across the entire wavelength range of 1.8 μm to 2.5 μm. An antireflection coating covering the 1.8 μm to 2.5 μm wavelength band is provided on the edge face of the on-chip spectral analysis chip.

[0034] The encapsulation and fixing structure 400 fixes the polarization-maintaining fiber 100 relative to the on-chip spectral analysis chip, so that the end of the tapered lens structure 200 is optically aligned with the wide end of the inverted tapered mode converter 300.

[0035] II. Two-stage joint mode field conversion structure This embodiment specifically describes a two-stage combined mode field conversion structure consisting of a tapered lens structure 200 and an inverted conical mode field converter 300.

[0036] like Figure 1As shown, the tapered lens structure 200 of the polarization-maintaining fiber 100 and the inverted conical mode converter 300 of the on-chip spectral analysis chip constitute a two-stage joint mode field conversion structure.

[0037] The tapered lens structure 200 performs the first-stage mode conversion. The diameter of the tapered transition region at the end of the polarization-maintaining fiber 100 gradually decreases along the light propagation direction, and the fundamental mode field size shrinks synchronously. Under the condition that the taper angle is less than the adiabatic tapering critical angle, the fundamental mode optical field energy adiabatically follows the change in the waveguide cross-section, gradually compressing the fundamental mode field of the polarization-maintaining fiber 100 from the micrometer scale (the original mode field diameter of the fiber is approximately 10 μm) to an intermediate mode field size (the mode field diameter at the taper tip is approximately 2-3 μm). Because the tapered transition region has a cutoff effect on higher-order modes, even if a small amount of higher-order mode components exist at the input end, they are effectively filtered out during the tapering process, ensuring the purity of the optical field mode output to the inverted tapered mode converter.

[0038] The inverted conical mode converter 300 performs the second-stage mode conversion. The mode field size at the wide end of the inverted conical mode converter 300 matches the output mode field size at the end of the tapered lens structure 200 (approximately 2-3 μm), and the width gradually decreases along the light propagation direction, further compressing the intermediate mode field size to match the submicron mode field size (approximately 0.5 μm) of the silicon waveguide.

[0039] After the two-stage transformation is cascaded, an equivalent gradual mode field size transition of one to two orders of magnitude from the fiber fundamental mode to the silicon waveguide fundamental mode is achieved. Through joint optimization of the two-stage gradual structure, the coupling efficiency is flattened across the entire wavelength range of 1.8μm to 2.5μm, and the difference in coupling efficiency between different wavelengths is effectively suppressed.

[0040] III. Encapsulation and Fixing Structure This embodiment specifically describes the composition and connection relationship of the encapsulation and fixing structure 400.

[0041] like Figure 2 As shown, the encapsulation and fixing structure 400 includes an optical fiber clamp 410, a chip stage 420, a rigid fixing member 430, and an elastic fixing material 440.

[0042] The fiber optic clamp 410 is used to fix the polarization-maintaining fiber 100. It is positioned near the chip end of the polarization-maintaining fiber 100 and is connected to the precision displacement stage.

[0043] The chip stage 420 is a copper stage used to support on-chip spectral analysis chips. The chips are mounted on the upper surface of the copper stage using thermally conductive materials.

[0044] The rigid fastener 430 is a Kovar alloy V-groove clamp, positioned at the end of the polarization-maintaining fiber 100 near the on-chip spectral analysis chip. The average coefficient of thermal expansion of Kovar alloy is approximately 5.2 × 10⁻⁶ °C over the range of room temperature to 150 °C. -6 / ℃, although slightly lower than the coefficient of thermal expansion of chalcogenide glass (As2S3) (approximately 20×10⁻⁶). -6 / ℃), but the difference between the two (approximately 15×10) -6 Under short fixed section (≤3mm) conditions, the relative thermal displacement caused by changes in ambient temperature (e.g., ±50℃) is approximately: ΔL = 3mm × 50℃ × (20-5.2)×10 -6 / ℃≈ 2.2μm. Although this value is slightly larger than the submicron level of purely rigid alignment tolerance, this invention, through the precise positioning of the Kovar alloy V-groove and the flexible constraint of the elastic fixing material 440, releases the stress actually transmitted to the chip end face in the micro-bend structure at the fiber tail, avoiding rigid displacement of the alignment point and thus ensuring the stability of coupling efficiency during temperature cycling. The rigid fixing member 430 has a dimension of less than or equal to 3mm along the fiber length direction. This short rigid section provides a precise mechanical reference and avoids end face damage caused by heat accumulation over a long length.

[0045] The elastic fixing material 440 is a low-modulus elastic material (such as silicone rubber) and is located in the region of the polarization-maintaining fiber 100 away from the on-chip spectral analysis chip (i.e., the region outside the rigid fixing section) to provide flexible constraint on the polarization-maintaining fiber 100 and release stress.

[0046] The copper stage and the Kovar alloy V-groove fixture are fixedly connected by Kovar alloy connectors.

[0047] A refractive index-matched curable adhesive 450 is disposed between the end face of the tapered lens structure 200 and the edge face of the on-chip spectral analysis chip. The curable adhesive 450 is transparent in the 1.8μm to 2.5μm wavelength range, is a low-shrinkage UV curable adhesive, and is a deuterated or halogen-substituted low-absorption UV curable adhesive (such as a specially formulated fluorinated acrylate) specifically used in the infrared band to suppress Fresnel reflection and optically couple the tapered lens structure 200 to the inverted conical mode converter 300.

[0048] IV. Fabrication Methods of Optical Interface Systems This embodiment provides a method for fabricating an optical interface system for a short-wave infrared on-chip spectrometer.

[0049] Step 1: Fabrication of polarization-maintaining fiber Preforms with elliptical fiber cores were prepared by a combination of melt quenching and extrusion.

[0050] First, high-purity As₂S₃ chalcogenide glass was selected as the core material, and As-S system glass with a slightly higher sulfur content was selected as the cladding material. The refractive index of the core glass was 2.42 (@2μm), and the refractive index of the cladding glass was 2.408 (@2μm). The raw materials were weighed according to the specified ratio and then placed into quartz ampoules. The ampoules were then sealed under a vacuum of better than 10...-3 The ampoule is sealed under Pa conditions. It is then placed in a gyratory furnace and heated to 650-750°C at a heating rate of 2-5°C / min, held for 12-24 hours, during which time the molten glass is thoroughly homogenized by gyratory motion. After melting, the ampoule is removed and quickly placed in an annealing furnace, held near the glass transition temperature (Tg≈185°C) for 2-4 hours, and then cooled to room temperature in the furnace to obtain a chalcogenide glass ingot.

[0051] The above-mentioned ingots are processed into rods suitable for extrusion, and then elliptical core preforms are prepared by extrusion. The core glass rods and cladding glass rods are respectively loaded into the core chamber and cladding chamber of the extrusion apparatus. Under inert gas protection, the extrusion apparatus is heated to approximately 300-400°C until the glass reaches an extrudable viscosity (approximately 10). 4 ~10 6 (Poisson). The core glass is extruded into shape using a die with an elliptical outlet, while the cladding glass is wrapped around the core through concentric ring outlets. The extrusion parameters are controlled so that the major axis radius of the elliptical core is approximately 6 μm, the minor axis radius is approximately 2 μm, and the cladding radius is approximately 40 μm.

[0052] The preform is mounted on the fiber drawing tower, and the heating temperature is set at approximately 350-450℃. During the drawing process, the drawing speed (approximately 0.5-5 m / min) and the rod feed speed are controlled to stabilize the fiber outer diameter at the target value. An online monitoring system is used to monitor the fiber outer diameter and the shape parameters of the elliptical core in real time, and feedback is used to adjust the drawing tension and temperature distribution to maintain the axial uniformity of the elliptical core shape. After drawing, a low-absorption protective coating (such as UV-cured acrylate or fluorinated polymer) suitable for use in the 1.8μm–2.5μm wavelength band is coated on the fiber surface, with a coating thickness of approximately 10-20μm.

[0053] The fabricated polarization-maintaining fiber 100 has an elliptical core cross-section and a mode birefringence greater than 10. -5 The operating wavelength range is 1.8μm to 2.5μm.

[0054] Step 2: Fabrication of the fiber end tapered lens structure The end of the polarization-maintaining fiber 100 is heated and stretched to form a tapered lens structure 200 with a tapered transition region.

[0055] Take the prepared polarization-maintaining fiber 100, remove the protective coating from its end about 3-5 cm long, and clean it with anhydrous ethanol. Fix the fiber to the precision displacement stage of the CO2 laser-heated fiber taper system.

[0056] A CO2 laser is used to locally heat the end of the polarization-maintaining fiber 100, with the heating temperature set at 200-300℃ (the softening temperature of As2S3 chalcogenide glass is approximately 185℃). A precision displacement stage is used to control the stretching speed, and the fiber end is tapered. During the tapering process, the output light power and output spot morphology are monitored in real time, and the stretching speed and heating temperature are controlled by feedback to ensure that the taper angle of the tapered transition region is less than the critical angle for adiabatic tapering (in this embodiment, the taper angle is controlled to be less than 5°). This allows the fundamental mode optical field energy to adiabatably follow the changes in the waveguide cross-sectional dimensions within the tapered transition region, suppressing leakage to higher-order modes or radiation modes.

[0057] After tapering, a focused laser beam is used to micro-shape the end of the tapered transition region, forming a microlens surface. The radius of curvature of the microlens surface is determined according to the convergence requirements of the emitted light spot. The end of the tapered transition region has a microlens surface.

[0058] Step 3: Fabrication of the on-chip inverted conical mode converter An inverted conical mode converter 300 is fabricated in the edge region of an on-chip spectral analysis chip.

[0059] An inverted conical waveguide was fabricated in the edge region of an on-chip spectral analysis chip using deep ultraviolet lithography and dry etching processes. The width of the inverted conical waveguide gradually widens from the narrow end near the chip interior towards the chip edge. The wide end dimension matches the output mode field size at the end of the fiber tapered lens structure 200, while the narrow end dimension matches the mode field size of the silicon waveguide in the on-chip spectral analysis chip. The length of the inverted conical waveguide sufficiently exceeds the minimum length required for wideband transmission, ensuring thermal insulation conditions are met across the entire 1.8μm–2.5μm band.

[0060] After the inverted conical mode converter 300 is fabricated, the edge face of the on-chip spectral analysis chip is chemically and mechanically polished to optical grade roughness (Ra<5nm), and a broadband antireflection multilayer film covering the 1.8μm to 2.5μm wavelength band is deposited by magnetron sputtering.

[0061] Step 4: Six-Dimensional Precision Alignment The prepared tapered lens structure 200 was mounted on a six-dimensional precision alignment platform, and the chip was fixed on the chip stage 420. The step resolution of the six-dimensional precision alignment platform reaches the sub-micron level.

[0062] A broadband light source or scanning laser source within the target operating band is injected into the polarization-maintaining fiber 100, and the coupled optical power and polarization extinction ratio are monitored simultaneously at the chip output. The point of highest coupling efficiency is found by scanning and translating the alignment position, while the fiber is rotated around its axis to align the polarization principal axis of the polarization-maintaining fiber 100 with the TE mode direction of the chip waveguide, so that the coupled optical power and polarization extinction ratio reach their optimal values ​​simultaneously.

[0063] After completing the joint alignment of three-dimensional position (X, Y, Z directions) and polarization principal axis (rotation around the fiber axis), the optimal alignment parameters are recorded.

[0064] Step 5: Encapsulation and Fixing After precise alignment, a low-shrinkage curing adhesive 450 with a matching refractive index is injected into the gap between the end face of the tapered lens structure 200 and the edge face of the on-chip spectral analysis chip. The curing adhesive 450 is transparent in the 1.8μm to 2.5μm wavelength range and has a refractive index between that of fiber-core glass and air, which is used to suppress Fresnel reflection.

[0065] UV curing is performed while maintaining alignment accuracy. During UV curing, the output power is continuously monitored to confirm that there is no displacement or deviation before curing is completed.

[0066] After UV curing, the polarization-maintaining fiber 100 is rigidly fixed at the end near the on-chip spectral analysis chip using a Kovar alloy V-groove clamp with a thermal expansion coefficient matching that of chalcogenide glass, forming a rigid fixing component 430. The Kovar alloy V-groove clamp has a dimension of less than or equal to 3 mm along the fiber length. This short rigid fixing section, in conjunction with the subsequent elastic fixing material 440, controls the relative displacement caused by temperature changes within the allowable range of coupling tolerance, while avoiding thermal stress concentration at the fiber-chip interface. The remaining areas of the polarization-maintaining fiber 100 are flexibly constrained using a low-modulus elastic fixing material (such as silicone rubber), forming the elastic fixing material 440, thus releasing stress. The on-chip spectral analysis chip is mounted on a copper stage using a thermally conductive material, forming a chip stage 420. The copper stage and the fiber clamp 410 are fixedly connected using Kovar alloy connectors.

[0067] The entire packaging process is completed in a dust-free and clean environment.

[0068] V. Shortwave Infrared On-Chip Spectrometer This embodiment provides a short-wave infrared on-chip spectrometer that includes the optical interface system described above.

[0069] The shortwave infrared on-chip spectrometer includes: an on-chip spectral analysis chip, an optical interface system, and a detector array.

[0070] The on-chip spectral analysis chip integrates an inverted conical mode converter 300 and a silicon waveguide. The end of the tapered lens structure 200 of the optical interface system is optically coupled to the wide end of the inverted conical mode converter 300. A polarization-maintaining fiber 100 receives the optical signal under test from an external light source or sampling probe, transmitting it to the on-chip spectral analysis chip in a low-loss, polarization-stable manner within the 1.8μm–2.5μm wavelength range. After being encoded and sampled by the on-chip spectral analysis chip, the optical signal is received by a detector array located at the output of the on-chip spectral analysis chip.

[0071] The preparation methods and parameters of each component of the optical interface system are the same as those in embodiments one to four, and will not be repeated here.

[0072] VI. Alternative Parameter Scheme This embodiment provides an alternative parameter scheme.

[0073] The core material of polarization-maintaining fiber 100 is a Ge-As-Se system chalcogenide glass with a core refractive index of 2.5-2.7, and the cladding material is a low-refractive-index glass of the same system (refractive index of about 2.4-2.6). The refractive index difference Δn between the core and cladding ranges from 0.01 to 0.03.

[0074] The major axis radius of the elliptical fiber core ranges from 5 to 8 μm, the minor axis radius ranges from 1.5 to 3 μm, and the cladding radius ranges from 30 to 50 μm.

[0075] The birefringence range of the mode is 1.5 × 10⁻⁶. -5 Up to 5×10 -5 .

[0076] The cone angle of the tapered lens structure 200 is controlled within the range of 3°-8°, the length of the tapered transition zone is within the range of 5-20mm, and the diameter of the cone tip is within the range of 2-5μm.

[0077] The inverted cone pattern converter 300 has a wide end width range of 2-5μm, a narrow end width range of 0.2-0.8μm, and an inverted cone length range of 100-500μm.

[0078] The rigid fastener 430 in the encapsulation and fixing structure 400 has a size range of 2-5mm along the length of the optical fiber, which is a Kovar alloy V-groove clamp.

[0079] All of the above-mentioned alternative parameter ranges can achieve the technical effects of the present invention. The specific optimal value is determined by simulation or experiment based on the actual working band and application requirements.

[0080] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An optical interface system for a short-wave infrared on-chip spectrometer, characterized in that, include: The polarization-maintaining fiber has a core material of chalcogenide glass and an elliptical core cross-section. The operating wavelength range of the polarization-maintaining fiber covers 1.8 μm to 2.5 μm. The mode birefringence of the polarization-maintaining fiber is generated by the geometric asymmetry of the elliptical core and is greater than 10. -5 ; A tapered lens structure is disposed at the end of the polarization-maintaining fiber. The tapered lens structure is formed by heating and stretching the end of the polarization-maintaining fiber to form a tapered transition region. The cone angle of the tapered transition region is smaller than the critical angle of the adiabatic tapered fiber, so that the fundamental mode optical field energy adiabatically follows the change of the waveguide cross-sectional size within the tapered transition region, suppressing leakage to higher-order modes or radiation modes. A microlens surface is formed at the end of the tapered transition region. An inverted conical mode converter is integrated into the edge coupling region of an on-chip spectral analysis chip. The width of the inverted conical mode converter gradually increases from the narrow end to the wide end along the light propagation direction. The mode field size at the wide end matches the output mode field size at the end of the tapered lens structure, and the mode field size at the narrow end matches the mode field size of the silicon waveguide in the on-chip spectral analysis chip. The encapsulation and fixing structure fixes the polarization-maintaining fiber relative to the on-chip spectral analysis chip, so that the end of the tapered lens structure is optically aligned with the wide end of the inverted conical mode converter.

2. The optical interface system according to claim 1, characterized in that, The chalcogenide glass is As₂S₃. The elliptical core has a major axis radius of 6 μm, a minor axis radius of 2 μm, a cladding radius of 40 μm, a core refractive index of 2.42, and a cladding refractive index of 2.

408. The polarization-maintaining fiber has a mode birefringence of 2.28 × 10⁻⁶ at a wavelength of 2 μm. -5 The birefringence of the mode is generated by the geometric asymmetry of the elliptical fiber core and does not depend on the heterogeneous stress region around the fiber core.

3. The optical interface system according to claim 1, characterized in that, The inverted conical mode converter is fabricated from silicon-based waveguide material using deep ultraviolet lithography and dry etching processes. The length of the inverted conical mode converter satisfies the adiabatic mode conversion conditions within the full wavelength range of 1.8μm to 2.5μm. The edge face of the on-chip spectral analysis chip is provided with an antireflection coating covering the 1.8μm to 2.5μm wavelength range.

4. The optical interface system according to claim 1, characterized in that, The tapered lens structure of the polarization-maintaining fiber and the inverted conical mode converter of the on-chip spectral analysis chip constitute a two-stage joint mode field conversion structure. The tapered lens structure performs the first-stage mode field conversion, gradually compressing the fundamental mode field of the polarization-maintaining fiber from the micrometer scale to the intermediate mode field size. The inverted conical mode converter performs the second-stage mode field conversion, further compressing the intermediate mode field size to match the submicrometer mode field size of the silicon waveguide. The two-stage joint mode field conversion structure achieves flattening of coupling efficiency across the entire 1.8μm to 2.5μm wavelength range.

5. The optical interface system according to claim 1, characterized in that, The encapsulation and fixing structure includes: Fiber optic clamps are used to fix the polarization-maintaining fiber; A chip stage for supporting the on-chip spectral analysis chip; A rigid fastener is disposed at the end of the polarization-maintaining fiber near the on-chip spectral analysis chip. The thermal expansion coefficient of the rigid fastener matches that of the chalcogenide glass. The dimension of the rigid fastener along the fiber length is less than or equal to 3 mm. The rigid fastener is a Kovar alloy V-groove clamp. An elastic fixing material is disposed in the region of the polarization-maintaining fiber away from the on-chip spectral analysis chip to provide flexible constraint on the polarization-maintaining fiber.

6. The optical interface system according to claim 5, characterized in that, The chip stage is a copper stage, and the copper stage is fixedly connected to the Kovar alloy V-groove fixture by a Kovar alloy connector. A refractive index-matched curing adhesive is provided between the end face of the tapered lens structure and the edge face of the on-chip spectral analysis chip. The curing adhesive is transparent in the 1.8μm to 2.5μm wavelength range and is a low-shrinkage UV curing adhesive, which is used to suppress Fresnel reflection and optically couple the tapered lens structure to the inverted conical mode converter.

7. A method for fabricating an optical interface system for a short-wave infrared on-chip spectrometer, characterized in that, Includes the following steps: Step 1: Fabrication of polarization-maintaining fiber. The core material of the polarization-maintaining fiber is chalcogenide glass, and the core cross-section of the polarization-maintaining fiber is elliptical with a mode birefringence greater than 10. -5 The operating wavelength covers 1.8μm to 2.5μm; Step 2: The end of the polarization-maintaining fiber is heated and stretched to form a tapered lens structure with a tapered transition region. The cone angle of the tapered transition region is smaller than the critical angle of the adiabatic tapered section, so that the fundamental mode optical field energy adiabatically follows the change of the waveguide cross-sectional size within the tapered transition region, suppressing leakage to higher-order modes or radiation modes. The output light power and output spot morphology are monitored in real time to provide feedback control of the tapering parameters. The end of the tapered transition region forms a microlens surface. Step 3: Fabricate an inverted conical mode field converter in the edge region of the on-chip spectral analysis chip. The wide-end mode field size of the inverted conical mode field converter matches the output mode field size at the end of the tapered lens structure, and the narrow-end mode field size matches the mode field size of the silicon waveguide in the on-chip spectral analysis chip. Step 4: Perform six-dimensional precision alignment between the end of the tapered lens structure and the wide end of the inverted conical mode converter to simultaneously achieve optimal values ​​for coupled light power and polarization extinction ratio; Step 5: Fix the polarization-maintaining fiber relative to the on-chip spectral analysis chip using the encapsulation and fixing structure. Inject a low-shrinkage curing adhesive with matching refractive index into the gap between the end face of the tapered lens structure and the edge face of the on-chip spectral analysis chip, and perform ultraviolet curing while maintaining alignment accuracy.

8. The preparation method according to claim 7, characterized in that, In step one, chalcogenide glass ingots are prepared by melt quenching, and then preforms with elliptical cores are prepared by precision extrusion molding. During the drawing process, the drawing tension and temperature distribution are controlled to maintain the axial uniformity of the elliptical core shape. After drawing, a low-absorption protective coating suitable for use in the 1.8μm~2.5μm wavelength band is applied.

9. The preparation method according to claim 7, characterized in that, In step two, a CO2 laser is used to locally heat the end of the polarization-maintaining fiber, and a precision displacement stage is used to control the stretching speed. The end of the tapered transition region is micro-shaped by a focused laser beam to form a microlens surface. In step three, an inverted tapered waveguide is fabricated using deep ultraviolet lithography and dry etching processes. The edge face of the on-chip spectral analysis chip is chemically and mechanically polished to optical-grade roughness. A broadband antireflection multilayer film covering the 1.8μm to 2.5μm wavelength band is deposited using a magnetron sputtering process.

10. The preparation method according to claim 7, characterized in that, In step five, the encapsulation and fixation further includes: after UV curing, rigidly fixing the end of the polarization-maintaining fiber near the on-chip spectral analysis chip using a Kovar alloy V-groove clamp with a thermal expansion coefficient matching that of the chalcogenide glass, wherein the dimension of the Kovar alloy V-groove clamp along the fiber length direction is less than or equal to 3 mm; flexibly constraining the remaining area of ​​the polarization-maintaining fiber using a low-modulus elastic fixing material; the on-chip spectral analysis chip is mounted on a copper stage using a thermally conductive material, and the copper stage and the fiber clamp are fixedly connected by a Kovar alloy connector.

Images